By Alexa Joy ShermanSometimes called “the stomach flu,” gastroenteritis can be caused by a variety of viruses, with rotavirus being the top contender for severe cases in children. But no matter what you call it, it’s never fun for babies or parents. “Prior to 2006, almost all kids got stomach bugs by 3 years old,” says Ari Brown, MD, a pediatrician in Austin, Tex., and coauthor of Expecting 411, Baby 411 and Toddler 411. Since then, a rotavirus vaccine has become available—but that doesn’t mean your baby won’t get an upset stomach. After all, only 44 percent of children received the shot during infancy, according to the 2009 National Immunization Survey, and there are plenty of other issues eager to trouble your baby’s tummy. How to spot itLots of diaper changes and laundry! “At best, your baby vomits once and has diarrhea for three to five days,” says Brown. “At worst, he vomits for 12 to 18 hours and has diarrhea 25 times a day for 10 days.” These symptoms may be accompanied by chills, clammy skin, excessive sweating and fever. How to deal Your top priority is offering small amounts of liquid to prevent dehydration. “The biggest mistake parents make is offering a huge amount of liquid as soon the baby throws up,” says Scott Cohen, MD, a Beverly Hills, Calif., pediatrician and author of Eat, Sleep, Poop: A Common Sense Guide to Your Baby’s First Year. “But their bellies aren’t ready, so they’re just going to throw it back up.” Don’t offer water, either. “The body needs electrolytes to replace what’s lost—sodium, potassium, sugar—and water actually induces more vomiting,” Cohen explains. So after waiting at least 30 minutes, simply nurse your baby (if you’re breastfeeding), or offer 2 to 4 ounces of fluid (expressed breast milk, formula or an oral electrolyte solution such as Pedialyte) every 30 to 60 minutes by spoon, syringe or bottle. And know this: As scary as first illnesses can be, they’re rarely cause for concern—and require little more than some TLC. However, nothing should trump your intuition. If you ever believe your baby’s symptoms suggest something serious, especially in those first few months, don’t hesitate to dial the doctor. More from New Parent: Cold and Flu Comfort for Babies